A copending U.S. patent application, Ser. No. 469,489 filed Feb. 24, 1983 in the name of Paulus et al for "Electret Microphone", and assigned to the assignee hereof and incorporated herein by reference, discloses an electret microphone comprising a metallic disc-shaped back electrode, an annular spacer washer extending in secured relation around the circumference of one surface thereof, and a circular electret diaphragm bonded around its circumferential border to the side of the washer away from the back electrode so that the diaphragm is maintained in spaced relation therefrom by the washer. The mentioned diaphragm is constructed of electret sheet material comprising a dielectrically-polarized radially-tensioned polymer film having a thin metal layer on its surface away from the washer. The polarization of the film creates between such layer and the back electrode an electric field which, when sound waves fall on the diaphragm, is modulated to produce between the metal layer and back electrode a varying voltage adapted to be transmitted from the microphone as an electrical signal representative of those sound waves.
In the course of manufacture of microphones of the sort described, their diaphragms are caused to be radially tensioned in a manner as follows: An electret sheet much larger than the diaphragms is secured by adhesive to the bottom of an annular holding disc so that the sheet stretches over the circular opening of the disc. The holding disc is placed over, and in coaxial relation with, an annular support disc having a smaller circular opening than the holding disc. The support disc bears on it a toroidal tensioning ring surrounding and concentric with that disc's opening and forming an annular upward projection on top of the support disc. To tension the electret sheet, the disc which holds it is moved down to press down on the support disc and, to cause the tensioning ring on the latter disc to (a) first contact a ring-shaped area of the sheet within the central opening of the holding disc and, then, (b) to press up against the sheet to upwardly displace (relative to the holding disc) a central sheet portion within such ring-shaped area. The sheet thereby is radially tensioned so that such central sheet portion is stretched across the tensioning ring. While that sheet portion remains so tensioned, a numerous plurality of the mentioned electret microphone washers are each bonded by adhesive to the tensioned central sheet portion so as, by the bonding, to fixedly retain in the part of the electret sheet extending across the washer opening the tensile stress than existing in the larger central electret sheet portion extending across the tensioning ring opening. Thereafter, the electret sheet is cut away around each washer to leave behind only that washer and the tensioned sheet material which is bonded to and extends across that washer, and which tensioned sheet material forms the diaphragm for the earlier described electret microphone.
In providing tensioned electret diaphragms by such method, it is not possible to selectively control during tensioning of the electrer sheet the amount of tension imparted to the electret sheet and later fixedly retained in the diaphragms made therefrom. The result is that the tension in the diaphragms may vary from one made batch thereof to another because of variation in the modulus of elasticity or other characteristic(s) of the electret sheets used in making successive batches of diaphragms. Such possible variation in diaphragm tension creates a problem, however, in that the diaphragm resonant frequency (which varies as a function of the diaphragm tension) is likewise not selectively controllable and, accordingly, may vary among various batches of diaphragms, but it is preferable that all diaphragms have the same resonant frequency.